1. Field of the Invention
The present invention relates to a marine pipeline installation tensioner, primarily for use in marine pipeline installation wherein a pipeline for transportation of hydrocarbons (oil, gas, etc) is laid on the seabed. The invention furthermore relates to a track unit for use in a marine pipeline installation tensioner
2. Description of Related Art
Tensioners are well known in the field of marine pipelaying, often having the task of supporting the weight of the launched pipeline. Tensioners are used for many pipelaying techniques, such as S-lay, Reel lay and J-lay. Tensioners have a large “load holding capacity” and in the field capacities of tens of tonnes, or even well over 100 tonnes, for a tensioner are not uncommon.
For example, the S-lay installation method relates to fabricating and paying out of a pipeline by repeated addition of new pipeline sections to the end of the pipeline being laid, the end of said pipeline being held aligned with a fabrication axis closer to horizontal than vertical.
One or more joining stations are positioned along a fabrication axis for joining new sections of pipe to the end of the pipeline. The pipeline is fed into one or more tensioners, the tensioners each defining a central pipeline transport axis aligned with the fabrication axis. The tensioners grip and guide the pipeline in a pipeline direction to the “stinger”. A “stinger” is a structure on the back of the vessel that supports the seagoing pipeline to control its bend radius. The stinger supports the pipeline in an arcuate path between the fabrication axis and a departure axis which may be substantially vertical.
To control movement of the pipeline, the tensioners exert a clamping pressure upon the pipeline. The tensioners thus hold the pipeline while laying it from the vessel to the seafloor.
In a known embodiment a marine pipeline installation tensioner comprises an upper and a lower track unit for gripping and guiding a pipeline. The track units are mounted in the tensioner frame such that the track units extend parallel to each other and define a central pipeline transport axis between them. The upper track unit and the lower track unit are positioned such that they define a “gap” between them, for receiving a pipeline. The track units are moveably mounted in the frame such that they can clamp a pipeline between them.
Typically, hydraulic cylinders are provided between the tensioner frame and each track units to moveably support the track units in the tensioner frame. The cylinders provide the “squeeze pressure” between the track units and the pipeline exterior needed to hold the pipeline on the basis of friction. The tensioner frame encloses the central pipeline transport axis and thus provides a closed frame in which the track units are supported. The closed frame supports the radially directed clamping forces generated by the tracks when gripping a pipeline, and thus enables the track units to exert a clamping pressure on the pipeline between them.
The cylinders furthermore allow for displacement of the track unit towards and away from each other to adjust the size of the gap between them. The gap can thus be adapted to different pipeline diameters, and to allow for a “widely opened” position of the track units wherein a large item (e.g. an accessory fitted in or on the pipeline, such as a pipeline connector) can pass through the tensioner.
Known marine pipeline installation tensioners are voluminous constructions. This is disadvantageous since the available deck space on vessels is limited, and is even more disadvantageous when the marine pipeline installation tensioner is to be installed below deck.